1. Field of the Invention
The present invention generally relates to wire bonding for a manufacturing process and, more particularly, ball wire bonding suitable for forming wire bonds at a fine pitch to integrated chips and package structures for fabrication of electronic devices.
2. Description of the Prior Art
Miniaturization of electronic devices provides not only manufacturing economies but provides performance enhancement by reducing signal propagation time between nodes of the electronic device. At the high performance levels which currently characterize modern digital circuits, design rules provide for extremely close spacing of components and connections therebetween. The vast majority of connections are therefore made on integrated circuit chips by lithographic processes as part of the integrated circuit design. However, many connections to integrated circuit chips are made by wire bonding. It is also possible, in view of the complexity and cost of manufacture of integrated circuits and modular packages, to repair connections on either by wire bonding.
In the past, wired connections have been accomplished by soldering or welding. At the current state of the art, however, many applications in which wire bonding is needed cannot tolerate either soldering or welding because of the size of the resulting bond or the heat budget of the device to which a connection must be made. Therefore, other forms of wire bonding have been developed.
Specifically, so-called thermal compression bonding has been developed in which a combination of heat and pressure causes a weld-like bonding of a wire to metallization on the chip or carrier substrate but with minimal, if any, melting of the metallization or wire. While thermal compression bonding produces high quality bonds, the process involves substantial temperature excursions and is slow; becoming economically impractical as the number of required wire bonds increases.
Ultrasonic bonding, sometimes referred to as wedge bonding, clamps a wire to a pad with a wedge or other structure which guides feeding of the wire and provides energy to complete the bond by ultrasonic vibration. This process may be conducted at a much higher repetition rate and much lower temperatures than thermal compression bonding. However, wire bonds formed by this technique are directional in that the axis of the wire is "wedged" parallel to the pad surface and obtaining the correct wire orientation in the plane of the pad surface is difficult and may require complex automated manipulations, including rotation of a mandrel-like transducer by which ultrasonic energy is applied and across which the wire is fed, which may slow the process significantly.
A third type of wire bonding to which the present invention is directed is referred to as thermosonic or ball bonding. This process preferably includes the application of some heat in addition to ultrasonic energy to form the bond, is far faster than wedge bonding because it is non-directional; a ball being formed on the end of the wire which will be attached to the chip so that the wire is oriented orthogonally to the chip as the bond is made. The other end of the wire bond is made to the carrier by wedge bonding as described above but since the wedge bond is formed subsequent to the wire bond to the chip, the direction of the wedge bond is established by the route of the wire, itself.
To reliably provide a volume of wire material located and formed such that it can be pressed against a pad with a so-called capillary through which the wire is dispensed, it is common to strike an electrical arc between the wire and another conductive surface completing a circuit much in the manner of electrical arc welding. The heat of the arc thus melts a small length of the wire which forms a so-called free air ball on the end of the wire. The cross-section of the free air ball in a direction orthogonal to the wire axis thus forms a flange-like feature which can be compressed against a pad by the capillary which also conducts ultrasonic energy to the ball for bonding with the bonding pad.
Therefore, it can be readily understood that the diameter of the free air ball determines the size of the contact area of the wire bond and can thus limit the pitch at which wire bonds can be formed. At the current state of the art a wire diameter of about 1.25 mils is used and the ball diameter developed is about 2.5 to 3.0 mils. Ball diameter is largely a function of the amount of energy in the arc that is struck which is roughly determined by the charging of a capacitor bank and the diameter of the wire which is the principal mechanism of conducting heat away from the ball.
Convection cooling of the ball and the wire is negligible and, during formation of the ball, the wire extends well past the capillary. The heat of the arc (and any resistive heating which may occur) is difficult to regulate from one free air ball formation to the next and therefore ball size will unavoidably vary within and beyond the range indicated above. Decrease of wire diameter will increase thermal resistance and increase variability of ball size with variability of arc heat. Further a threshold amount of energy will be required to reliably strike an arc to form the free air ball and this threshold cannot be scaled with wire diameter. Therefore, average minimum size as well as the variability of size of the free air ball may increase with decreasing wire size.
It also follows that the temperature of a region of the wire adjacent to the free air ball may rise very close to the melting point of the wire material. Such heating causes annealing of a region of the wire adjacent to the free air ball which extends for some distance along the wire (a few microns) for a 1.25 mil (0.00125 inches or about 32 microns) wire but longer for smaller diameter wire. Annealing causes an increase in the size of metal grains in the grain structure of the wire and relieves internal stresses in the material of the wire, effectively causing the wire to become somewhat softer (e.g. more malleable) in a region called the heat affected zone (HAZ) over which some degree of annealing takes place. Further, annealing may cause the grain size to approach 1 mil which infers that a few grain boundaries may cover a significant portion of the cross-sectional area of a 1.25 mil wire within the HAZ and potentially the entire cross-section of the wire if wire diameter is reduced. It can thus be understood that annealing can also cause substantial weakening of fine wires which could then be broken by relatively small tensile forces which can occur during subsequent assembly processes.
Softening of the wire in the heat affected zone allows the wire to be more easily deformed (and aggravates reduction of wire stiffness as wire diameter is reduced). Thus the geometry of the wire routing is less readily controllable both during formation of the connection and thereafter until the wire bond is encapsulated to complete the packaging of the electronic device. Such deformations are generally referred to as sag (in a direction perpendicular to the connection surface), sweep (in a direction generally parallel to the bond surface) and "s-ing" which may be visualized as a deformation similar in shape to deformation caused by buckling under axial compression.
Since connections formed by wire bonding may cross and wire bonds may be formed closely adjacent each other, any of these deformations, alone or in any combination may potentially cause shorting between wires of respective connections. Insulation cannot be provided on the wire without contamination of the bonding surfaces. Therefore, wire bond connections may become shorted together as formed and are subject to damage due to even relatively slight accelerations or inadvertent contact with other structures before encapsulation can be performed or even due to viscous drag of applied materials during the encapsulation process itself.
As a partial solution in the past, the connections were formed in relatively tall loop configurations in which the portion of the connection which is not annealed is increased and additional path width is provided for flow of encapsulation materials. However, such a tall loop configuration actually increases the likelihood of shorting as wire bond connections are made closer together. Further, the mechanical advantage of forces from flow of encapsulating material is increased by the height of the loop and, while marginally tolerable for 1.25 mil wire, would cause collapse of such loops formed by thinner wire especially when made more malleable and weaker in the heat affected zone. Moreover, a greater height of encapsulation may compromise other processes such as package heat dissipation, post-assembly cleaning, solder connection pitch or second level attachment processes such as ball grid array (BGA) techniques.
It is important to appreciate that the integrated circuit chip or chips constitutes, by far, the most expensive portion of any electronic circuit package which may contain one or more of them. The economic advantage of miniaturization is largely based on the number of chips which can be fabricated on a wafer with the same set of process steps. Nevertheless, the functionality of any chip is limited by the capacity for conducting signals to it or away from it. Therefore, the potential for reduction of chip area is often limited by the area occupied by connection pads and wire bonding pads in particular.
Typical wire bonds can currently be reliably achieved at a pitch (e.g. center-to-center distance) of about 100 microns. However, for a given number of wire bond connections, a twenty percent reduction in pitch corresponds to about a one-third reduction in required chip area and cost of an individual chip. Conversely, for a given chip area, a twenty percent reduction in wire bond pitch corresponds to an increase in the number of wire bonds which can be formed of about fifty percent; corresponding to a substantial increase in the potential functionality of the chip. However, even for smaller reductions in wire bond pitch (e.g. to less than 90 microns) it is evident that more stringent tolerances will be required for free air ball diameter than can presently be produced by known techniques and that smaller diameter wire will be required which will increase the difficulty of obtaining uniform and reduced dimensions of free air balls. Reduced wire size will also tend to increase the size of the heat affected zone in the wire while making the malleability and weakness of the wire substantially more critical to manufacturing yield. Nevertheless, the functionality of chips which can be developed at the present and foreseeable state of the art will require wire bond pitches of 60 microns or less at which the size and uniformity of free air ball diameter and extent of the HAZ will be extremely critical and beyond the capability of current techniques notwithstanding the fact that significantly smaller wire diameter will be required.